Thread length and forum performance

I had a question about best practices for actually running a forum. On my boards I’ve got several topics which are getting exceptionally long (25+ pages). On other forums I frequent, I’ve seen that moderators often tend to close long threads and open a new topic as a continuation of the discussion. I assume this is considered a best practice for some performance reason. Should I consider adopting the same approach, or are bbPress topics/replies queried in a way that won’t be bothered by lengthy threads?

If I should clip threads at a certain length, what’s a good ballpark number of replies at which to split the thread?